Coliform bacteria are a diverse group of metabolically-related related microorganisms. The group is defined by their ability to ferment lactose, a disaccharide found in milk, to a group of metabolic byproducts that include carbon dioxide gas. Thus, the ability to directly and accurately enumerate coliform bacteria in sample depends upon the ability to detect gas production by isolated colonies of the bacteria.
Thin-film culture devices are conveniently used in place of traditional microorganism enumeration techniques that use, for example, Petri dishes containing agar media. Several examples of such thin-film culture devices are described in U.S. Pat. No. 4,565,783 to Hansen et al. and U.S. Pat. No. 5,364,766 to Mach et al.; which are both incorporated herein by reference in their entirety. In a typical device reported by Hansen et al., a cold-water soluble dry powder containing a gelling agent and microbial growth nutrients is coated on a waterproof substrate. A transparent, read-through cover sheet coated on a surface with an acrylate adhesive containing an indicating dye and powdered gelling agent are attached to the coated substrate.
When the device is used, a predetermined amount of an aqueous sample is typically placed in contact the coated substrate and the cover sheet is placed over the sample and substrate. The aqueous sample hydrates the soluble dry powder which then forms a gelled medium capable of sustaining microbial growth. During the growth period, the indicator dye adhered to the cover sheet reacts in the presence of viable microorganisms to give a detectable response that allows visualization of bacterial colonies which are grown on the culture device.
For the detection of gas-producing bacteria (e.g., CO2-producing coliform bacteria), the construction of the Hansen et al. devices advantageously causes the biogenic gas to be trapped adjacent the gas-producing colony in the gelled medium between the substrate and the coversheet.